THE OLDEST MOUNTAIN IN ENGLAND. 
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Edge ; but it must not be supposed that the beds are so regular 
and persistent. The limestone of Wenlock Edge is continuous 
and unbroken for nearly twenty miles ; and in the United 
States, the Corniferous Limestone extends from the Hudson to 
the Mississippi. But the lavas and ashes of the Wrekin form 
lenticular masses rather than regular beds. It is rarely possible 
to follow a stratum along its line of outcrop for many hundreds 
of yards. The breccias especially thin out very rapidly. But 
this is precisely what the analogies of modern volcanic action 
would lead us to expect. The layers of lava and tuff of which 
a cone is composed, and by which it is surrounded, thin out 
rapidly towards the circumference of the volcano, and the 
coarser the material the less persistent is the bedding. Volcanic 
matter ejected under the sea, as was the case with some, if not 
all, of the Wrekin beds, is deposited more evenly and over a 
much wider area ; but the principle is the same. 
Associated with the stratified rocks of which the mountain is 
chiefly composed, are certain masses of much less antiquity, to 
which the term “ greenstone ” may appropriately be applied. 
In ascending the Wrekin from the north-east end, we notice in 
the centre of the ridge, about midway between the north-east 
end and the summit, a bare rounded hump, which on examina- 
tion proves to be a mass of greenish dolerite or basalt, which 
has been forced up when in a molten state through the midst of 
the older bedded series, disturbing their strike, and apparently, 
in one place, reversing their dip. From this disruptive boss 
proceed several vertical dykes of basalt, three of which can be 
well seen at the north-east end of the mountain, striking 
towards the central mass. This basaltic rock was probably 
erupted in newer Palaeozoic times, but its exact age does not 
affect our present inquiry. The Wrekin existed as a distinct 
elevation many epochs before the dolerite boss and dykes were 
formed, and their bulk is unimportant compared with the mass 
of the mountain. 
The Wrekin, though mainly, is not exclusively composed of 
igneous rocks. Buttressing the ridge on both sides are certain 
beds of quartz rock, about two hundred feet thick, dipping away 
from the axis at an average angle of 45°. They also lap round 
the ends, and their dip is, in every case, away from the moun- 
tain. This quartzite evidently once formed a continuous bed of 
sandstone, and, judging from the dip of the pre-existing volcanic 
strata, was deposited unconformably upon them. Then an up- 
heaving force produced two cracks in the rigid crust of bedded 
felspathic rock, and pushed up between the cracks a narrow wedge, 
which, in its turn, forced a way up through the sandstone, tilting 
it off on all sides, and crushing up its lower surface into a breccia. 
